Simulation of Nocturnal LLJs with a WRF PBL Scheme Ensemble and Comparison to Observations from the ARM Project

Low Level Jets (LLJs), a common weather phenomenon occurring in the Plains states in 47% of all soundings in the warm season and 45% during the cold season (Whiteman 1997), have long been known to play an important role in the evolution of mesoscale convective systems and are now becoming increasingly more important in the study of wind power. LLJs can be described as regions of moderately strong winds in the lower atmosphere with roughly 50% of the peak winds occurring below 500m (Whiteman 1997). We have simulated LLJs using an ensemble of 10km grid spacing versions of the Weather Research and Forecasting (WRF) model with six different Planetary Boundary Layer (PBL) schemes. Two locations, Lamont, OK and Beaumont, KS, within the U.S. Department of Energy Atmospheric Radiation Measurement (ARM) project were used to validate the ensemble. Locations were chosen based on data availability below 500m with a vertical resolution of 60m below 2,462m. Thirty cases were chosen from June 2008 to May 2010 and compared to model output at both sites. The dates were chosen based on a nocturnal LLJ being present at both the Lamont and Beaumont sites, with a mix of strong and weak LLJ cases being used. In addition to comparing the simulations of the LLJ among the different model configurations, we will present a climatology of the height of maximum intensity and peak magnitude of the observed jets for all 30 cases. Preliminary results suggest substantial differences in the simulation of the LLJs depending on which PBL scheme is used. These differences are noticeably greater than those occurring at levels closer to the ground, such as 80m, a common height of wind turbines today. The results suggest that forecast uncertainty will increase as taller turbines are used for wind power generation.

References: Whiteman, C. D., X. Bian, and S. Zhong, 1997: Low-level jet climatology from enhanced rawinsonde observations at a site in the Southern Great Plains. J. Appl. Meteor., 36:1363–1376.